As solid-state imaging devices (image sensors) using photoelectric conversion elements detecting light and generating a charge, CCD (charge coupled device) image sensors and CMOS (complementary metal oxide semiconductor) image sensors have been put into practical use. CCD image sensors and CMOS image sensors have been widely applied as parts of digital cameras, video cameras, monitoring cameras, medical endoscopes, personal computers (PC), mobile phones and other portable terminals (mobile devices) and other various types of electronic apparatuses.
CCD image sensors and CMOS image sensors use photodiodes for photoelectric conversion elements, but differ in the methods of transferring the photoelectrically converted signal charges. In a CCD image sensor, a signal charges are transferred to an output part by a vertical transfer part (vertical CCD, VCCD) and horizontal transfer part (horizontal CCD, HCCD) then converted to electrical signals and amplified. Contrary to this, in a CMOS image sensor, charges which are converted for each pixel including a photodiode are amplified and are output as a read-out signal.
Each pixel in a CMOS image sensor is for example configured by including as active elements, for one photodiode, four elements of a transfer element constituted by a transfer transistor, a reset element constituted by a reset transistor, a source follower element (amplification element) constituted by a source follower transistor, and a selection element constituted by a selection transistor (see for example PLT 1). Further, each pixel may be provided with an overflow gate (overflow transistor) for discharging an overflow charge overflowing from the photodiode in an accumulation period of the photodiode.
The transfer transistor is connected between the photodiode and an output node constituted by a floating diffusion layer (FD). The transfer transistor is held in a non-conductive state in the charge accumulation period of the photodiode. In the transfer period transferring the accumulated charge in the photodiode to the floating diffusion, a control signal is supplied to the gate whereby it is held in a conductive state and transfers the charge photoelectrically converted in the photodiode to the floating diffusion FD.
The reset transistor is connected between a power supply line and the floating diffusion FD. The reset transistor, when given a reset-use control signal at its gate, resets the potential of the floating diffusion FD to the potential of the power supply line.
The floating diffusion FD is connected the gate of the source follower transistor. The source follower transistor is connected through the selection transistor to the vertical signal line and configures a source follower together with a constant current source of a load circuit outside of the pixel part. Further, a control signal (address signal or select signal) is given to the gate of the selection transistor, whereby the selection transistor turns on. When the selection transistor turns on, the source follower transistor amplifies the potential of the floating diffusion FD and outputs a voltage in accordance with that potential to the vertical signal line. Through the vertical signal line, voltages output from the pixels are output to a pixel signal readout circuit constituted by a column-parallel processing part.
Further, in each pixel, as the photodiode (PD), a pinned photodiode (PPD) is widely used. On the substrate surface forming the photodiode (PD), there is a surface level due to dangling bonds or other defects, therefore a large charge (dark current) is generated by the heat energy, so a correct signal can no longer be read out. In a pinned photodiode (PPD), a charge accumulation part of the photodiode (PD) is buried in the substrate, so it becomes possible to reduce entry of dark current to the signal. Note that, the sensitivity of a photodiode (PD) can be changed by for example changing an exposure time etc.
The pinned photodiode (PPD) is for example configured by forming an n-type semiconductor region and forming a shallow p-type semiconductor region which has a rich impurity concentration for suppressing dark current on the surface of this n-type semiconductor region, that is, in the vicinity of the interface with an insulation film.